The present invention relates to a cold-forged shaft having at one end thereof a shaped portion of a gear, serrations or the like, such as that used for an armature shaft in a starter motor of an internal combustion engine or the like, and to a method of and an apparatus for manufacturing the shaft.
Generally, as shown in FIG. 16 by way of example, an armature shaft 2 for use in a starter motor 1 of an internal combustion engine is provided at one end thereof with a gear 3 for connection to a crankshaft of the engine via a one-way clutch.
In the case of this armature shaft 2, forming the outer configuration thereof and forming the gear 3 have generally been performed by cutting. However, an attempt has been made to form them by cold forging for simplicity of manufacture.
The cold forging hitherto attempted is carried out in such a manner as shown in FIGS. 17 and 18.
First, a bar member W1 as shown in FIG. 17 is prepared as a material. The bar member W1 is squeezed by forward extrusion with a forging die to form a shaft blank W2 which has a small diameter portion 4a and a large diameter portion 4b as shown in FIG. 18. Then, by means of another die, the small diameter portion 4a is squeezed again to form the small diameter portion 4a of uniform outer size, as shown in FIG. 19. At the same time, one end of the large diameter portion 4b is squeezed by a slight amount to form a flange portion 4c, and the gear 3 is formed to extend for a predetermined length from the other end of the large diameter portion 4b toward the flange portion 4c. Thus, the armature shaft 2 is obtained.
In the conventional armature shaft 2 described above, however, the following drawback arises.
Namely, in shaping the armature shaft by cold forging as described above, a bend may sometimes be caused in the armature shaft 2 during heat treatment after the shaping. This bend has to be corrected after the shaping. When correcting the bend, however, since the shaft changes drastically in shape at a continuous connection between the small diameter portion 4a and the flange portion 4c, the stress due to a correction force concentrates at the continuous connection, resulting in the drawback that breakage of the shaft or the like occurs.
Furthermore, when the starter motor 1 starts operating, the armature shaft 2 has a reaction force from the crankshaft, and torsion of the shaft results. A torsional force and torsional vibrations at this time also cause stress concentration to occur at the continuous connection between the small diameter portion 4a and the flange portion 4c in like manner as in the above described case. Accordingly, there is a fear that reduction in the strength of the armature shaft 2 may be brought on.